Trace metal-humate interactions. I. Experimental determination of conditional stability constants

The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca2+, NpO2+, Co2+, Ni2+, UO22+ and Eu3+ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co2+,UO22+ and Eu3+ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca2+ congruent to NpO2+ < Co2+ < Ni2+ < UO22+ < Eu3+. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical 'rereview' of the auxiliary stability constants used for the calculation of the equilibrium coefficients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co2+, UO22+ and Eu3+. For Ni2+, Ca2+ and NpO2+ this effect was less pronounced. For all metal ions tested, the influence of ionic strength was of less importance, and the conditional stability constants did not show a significant dependence on the type of humic substances investigated. (C) 2000 Elsevier Science Ltd. All rights reserved.